1. Field of the Invention
This invention relates to an air charge measuring system for fuel injection engines.
2. Prior Art
In an electronic fuel control system for a fuel injection engine, the amount of fuel to be injected is calculated based upon the intake airflow. It is thus desirable to have an accurate method for obtaining the intake airflow in order to have a better control over the air fuel ratio. Both airflow metering systems and airflow calculation systems are known. The former measures the airflow directly by using an airflow sensor such as a vane meter or a hot wire-type mass airflow meter. The latter measures the airflow indirectly by calculating it based on engine operating conditions. One known airflow calculation system is the so-called speed density system in which the airflow is calculated based upon the intake manifold pressure and the engine speed. Another known airflow calculation system is the throttle angle-pressure method in which the airflow is calculated based upon the angle of the throttle in the throttle bore and the ratio of the intake manifold absolute pressure of the engine to the atmospheric pressure.
It is also known that each method mentioned above has advantages and disadvantages. Therefore, a number of systems which incorporate two different methods have been proposed. In these known hybrid systems, the more accurate method to measure the airflow is selected based upon the engine operation conditions. This results in better air fuel ration control, improved fuel efficiency, and reduced undesirable exhaust gases.
The intake airflow per intake stroke, which is the intake air charge, has to be obtained to calculate the amount of fuel to be injected. Therefore, instead of obtaining the intake airflow, one can measure the intake air charge to calculate the fuel pulse width. The intake air charge can be obtained by using the throttle angle information and the engine speed information. This is the so called speed-throttle method. One method of obtaining the air charge in a speed-throttle system is to use an air charge look up table which is obtained experimentally as a function of the engine speed and the throttle angle. The air charge can also be obtained by integrating the instantaneous airflow measured by an airflow meter such as the mass airflow meter over one engine intake stroke period. The instantaneous mass airflow is directly measured and obtained by sampling the sensor at a fixed-time interval, e.g., 1 ms. This is a mass airflow system.
As in the case of the airflow measuring systems, both air charge measuring systems have their own merits and disadvantages. In the mass air system the airflow and thus the air charge obtained are more accurate because they are directly measured. However, the accuracy is limited by the characteristics of the sensor. To be more specific, it is accurate only when the airflow is not too high. The speed-throttle system costs less, because there is no need for a mass airflow sensor, and has a broader operating range. On the other hand, the air charge obtained is less accurate since it is obtained indirectly from the engine speed and the throttle angle. In some applications where a large range of RPM operating capabilities is required, the mass airflow readings obtained at high RPM's may be much lower than the actual mass airflow because of the sensor's characteristics. In this case, the air charge obtained using the engine speed and the throttle position information is more accurate.
It would be desirable to have a hybrid system which combines the mass air system and the speed-throttle system so that the most appropriate method can be selected under all engine operating conditions to obtain the most accurate air charge. These are some of the problems this invention overcomes.
Hybrid systems are known. For example, U.S. Pat. No. 4,644,474 issued to Apposchanski et al teaches selecting between the more accurate of two airflow determinations. One determination measures a parameter characterizing airflow into the engine that has an adaptive correction. Another calculates airflow into the engine as a function of engine speed and air density and has an adaptive correction. The patent teaches determining both airflows before deciding which airflow to use. This clearly necessitates at least one redundant airflow determination.
U.S. Pat. No. 4,773,375 issued to Okino et al teaches fuel injection control based on an intake airflow rate sensing system and fuel injection control based on a speed density system depending upon the amount of intake air.
U.S. Pat. No. 4,664,090 issued to Kabasin teaches a system for measuring the airflow into the engine using a pair of airflow measuring concepts selectively enabled dependent upon engine operation so as to accurately achieve a measurement of airflow over the full range of engine operation. The patent teaches measuring airflow utilizing speed density and a throttle angle pressure methods by selectively employing each of the methods in engine operating regions at which it is best suited for air measurement.